As the use of new renewable energy is rapidly increasing, a demand for an energy storage system using a battery is rapidly increasing. As such batteries, a lead battery, a nickel/hydrogen battery, a vanadium battery and a lithium battery may be used. However, since the lead battery and the nickel/hydrogen battery have very low energy density, they require larger space for storing the same amount of energy. In addition, the vanadium battery has the problems in that it causes environmental pollution due to the use of a heavy metal-containing solution, and performance degradation due to the immigration of a small amount of a material between an anode and a cathode through a membrane separating the anode and the cathode, and thus, has not been commercialized on a large scale. The lithium battery having excellent energy density and output characteristic is technologically advantageous, but not economical for use as a secondary battery for large-scale power storage due to resource scarcity of a lithium material.
In order to solve the foregoing problems, many attempts have been made to use sodium which is a rich resource on earth, as a material of the secondary battery. Among those, as disclosed in U.S. Patent Application Publication No. 20030054255, a sodium-sulfur battery using beta alumina having selective conductivity for sodium ions, and having an anode impregnated with sodium, and a cathode impregnated with sulfur, is currently being used as a large-scale power storage device.
However, as to the existing sodium-based secondary battery such as the sodium-sulfur battery or a sodium-nickel chloride battery, considering the conductivity and melting points of the battery components, the sodium-nickel chloride battery has an operating temperature of 250° C. or more, and the sodium-sulfur battery has an operating temperature of 300° C. or more. Due to such problems, those batteries are disadvantageous in economic aspect in manufacture or operation for reinforcement of temperature maintenance, sealability maintenance and safety.
In order to solve the foregoing problems, a room temperature-type, sodium-based battery is being developed, but has a very small output, and much less competitive than a nickel-hydrogen battery or a lithium battery.